The present invention relates to an active matrix type display device employing light emitting devices such as EL (electro-luminescence) devices or LEDs (light emitting diodes) each of which emits light by causing a driving current to flow through a light emitting thin film such as an organic semiconductor thin film, and thin film transistors for controlling the light emitting operation of the respective light emitting devices.
In recent years, as the advanced information society has come, there has been increasing demands for personal computers, portable information terminals, information communication apparatuses or complex products thereof. A thin and light-weight display device is suitable for these products, and hence the liquid crystal display device or the display device constituted by the self-light emitting type EL devices or the LED devices. The self-light emitting type display device of the latter has the features that the visibility is excellent, the visible angle characteristics are wide, it is suitable for the moving pictures since it is excellent in the high speed response, and so forth, and hence it is expected that the self-light emitting type display device will be important more and more in the information communication field in the future. In actual, recently, the rapid enhancement of the light emitting efficiency of the organic EL device or the organic LED device (hereinafter, the OLED is the general form for these devices) in which the organic material is used as the light emitting layer, and the advance of the network technology for making the image communication possible are combined to make the expectation to the OLED display device go on rising.
An example of the OLED display device according to the prior art is described in Pioneer RandD Vol. 8, No. 3, pp. 41 to 49. In accordance with this example, as shown in FIG. 6A, OLEDs are respectively arranged in the intersections of n anodes 61 which extend longitudinally and m cathodes 62 which extend transversely to form a simple matrix in which pixels P11, . . . , Pmn are provided. Then, each of the anode lines is driven by a constant current voltage-source 63 every cathode line to scan the cathode lines in the line-at-a-time manner. In such a way, the time division driving is carried out. Each of the pixels can be expressed in the form of an equivalent circuit shown in FIG. 6B, in which a parasitic capacity 65 is parasitically connected in parallel with an OLED 64. The value of this parasitic capacity 65 is so large as to be about 20 pF in the square of 0.3 mmxc3x970.3 mm, and hence in order to obtain the desired picture quality by the time division driving requiring the high speed as described above, it is necessary to devise the driving waveform for which the charge and discharge of the electric charges to and from the parasitic capacity are taken into consideration. In actual, in the above-mentioned prior art, there is adopted the complicated driving method wherein the timing in which all of the electrodes are grounded once is provided.
Instead of the above-mentioned simple matrix, the active matrix driving in which TFTs are provided in the pixels, respectively, has also been studied. The technology for manufacturing the OLED display device in the form of the active matrix structure to drive the same, for example, is disclosed in JP-A-8-241048 and U.S. Pat. No. 5550066, and also in WO98/36407 in which the contents of the driving voltage are described in more detail. For the typical pixels of the OLED display device of the active matrix system thus disclosed, as shown in FIG. 7, the light emission luminance of the OLED 76 is controlled by the active device driving circuit constituted by at least two TFT switch transistor Tsw73 and driver transistor Tdr74, and one storage capacitor 75. More specifically, the voltage corresponding to the electric charges which are stored in the storage capacitor 75 through the switching transistor 73 provides the gate voltage of the driver transistor 74, and the OLED 76 is driven by the current which is determined on the basis of the gate voltage. However, in actual, there arises the problem that the ununiformity of the display picture quality is generated due to the ununiformity of the threshold voltage and the charge drift mobility of the driver transistor.
As for the system having the possibility of clearing the above-mentioned two problems, as shown in FIG. 8, the active matrix system of providing one transistor in one pixel to carry out the driving is disclosed in JP-A-4-125683.
In the one pixel-one transistor system disclosed in the above-mentioned prior art, it is possible to realize the uniform display characteristics on the basis of the simple pixel structure and driving method. However, since the light emission time of the pixels of this system is equal to that of the simple matrix system, the current value must be increased. While under such a situation, the means for ensuring the reliability of the device is required, any of the effective techniques therefor has not yet been disclosed.
According to the present invention, there is provided an (OLED display device in which a single switch transistor is provided in each of pixels, and a constant current-voltage source is connected to the outside of a panel in order to carry out the driving, wherein in order to reduce the degradation of the luminance characteristics due to the flowing of a large current through the OLED, the voltage scheme is adopted in which in the conduction of the switch transistor, a reverse bias is applied to the OLED, and a driving waveform is provided in which the reverse bias is held in the non-conduction of the switching transistor. In addition, in order to reduce the level of a momentary current which is caused to flow through the OLED, a ramp wave or a square wave is applied to one side electrode of a storage capacitor to provide a driving waveform in which a current contributing to the light emission is caused to flow even in the non-conduction of the switching transistor.
According to one aspect of the present invention, there is provided an organic LED display device including: thin film transistors in which a plurality of gate lines and a plurality of data lines intersecting the plurality of gate lines are provided on a substrate, pixels are defined by the plurality of gate lines and the plurality of data lines, and a gate scanning signal is applied to the pixels through the gate lines, respectively; and light emitting devices each of which emits light by a driving current, which is caused to flow between an associated one of pixel electrodes formed in correspondence to the pixels and an associated one of counter electrodes opposite to the respective pixel electrodes, in accordance with a data signal which is supplied from the associated one of the data lines synchronously with a timing when the associated one of the thin film transistors becomes the conduction state, wherein each of the light emitting devices is an organic LED device, and for a part of a period of time when the associated one of the thin film transistors is in the non-conduction state, the associated one of the organic LED devices is in the non-light emission state, and also a bias having the polarity reverse to that in the light emission is applied thereto.
According to another aspect o the present invention, there is provided an organic LED display device including: thin film transistors in which a plurality of gate lines and a plurality of data lines intersecting the plurality of gate lines are provided on a substrate, pixels are defined by the plurality of gate lines and the plurality of data lines, and a gate scanning signal is applied to the pixels through the gate lines, respectively; and light emitting devices each of which emits light by a driving current, which is caused to flow between an associated one of pixel electrodes formed in correspondence to the pixels and an associated one of counter electrodes opposite to the respective pixel electrodes, in accordance with a data signal which is supplied from the associated one of the data lines synchronously with a timing when the associated one of the thin film transistors becomes the conduction state, wherein each of the light emitting devices is an organic LED device, each of storage capacitors is connected in parallel with the associated one of the organic LED devices, electrodes of the associated ones of the storage capacitors are connected to a common electrode every row, the common electrode is connected to a power source different from that of common electrode of the organic LED devices, and for a part of a period of time when the associated one of the thin film transistors is in the non-conduction state, the associated one of the organic LED devices is in the non-light emission state, and also a bias having the polarity reverse to that in the light emission is applied thereto.